U.S. Embassy to Russia press attache Joseph Kruzich told Interfax on Thursday they had expressed concern to Russian Foreign Ministry Ombudsman for Human Rights, Democracy and Rule of Law Konstantin Dolgov and asked for careful investigation of the issue.

The diplomat said the U.S. was concerned over a series of arrests and measures taken against the participants in the May 6 rally and opposition members, among them Alexei Navalny, Sergei Udaltsov, Konstantin Lebedev and Leonid Razvozzhayev.